Unstable optical resonator and laser device

ABSTRACT

An unstable optical resonator for an optically active medium comprising a spherical back mirror and a spherical outcoupling mirror is proposed, and an outcoupling which is asymmetrical in relation to the optical axis takes place therein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.11/061,380, filed Feb. 18, 2005 now U.S. Pat. No. 7,415,057, whichclaims the benefit of German Application No. 10 2004 008 640.0, filedFeb. 21, 2004, which are incorporated herein by reference in theirentirety and for all purposes.

FIELD OF THE INVENTION

The invention relates to a resonator for an optically active medium. Theinvention further relates to a laser device and, in particular, a gaslaser device with such a resonator.

BACKGROUND OF THE INVENTION

Optical resonators are known, for example, from the publication“Optische Resonatoren für Hochleistungs-Festkörper-Laser” (OpticalResonators for high-power solid-state Lasers) by N. Hodgson,Festkörper-Laser-Institut Berlin GmbH, internal report, December 1990 orfrom the article “Analysis of stable-unstable free electron laserresonators” by C.-C. Shih, SPIE Vol. 1868, pages 278 to 285. The article“Improvement of slab-laser beam divergence by using an off-axisunstable-stable resonator” by K. Kuba et al. (Optics Letters, Vol. 15,1990, pages 121 to 123) also discloses resonators. In particular,cylindrical mirrors are used in the optical resonators disclosed inthese references.

SUMMARY OF THE INVENTION

In accordance with the invention, a resonator is provided which isusable for optically active media with small gain and large crosssection. In accordance with the invention, an unstable optical resonatorfor an optically active medium comprises a spherical back mirror and aspherical output mirror, and an outcoupling which is asymmetrical inrelation to the optical axis takes place therein. A purely unstableresonator is made available by the use of spherical mirrors. Formationof a high multimode in a stable direction, as in the case of cylindricalmirrors, is thereby prevented. This, in turn, results in atime-independent mode, and an intensity distribution which is not toohighly structured is also provided in the far field. In comparison withcylindrical mirrors, the inventive resonator with the spherical mirrorsis considerably easier to adjust, and it is also considerably lesssensitive to deviations in mirror radii and tiltings.

The resonator according to the invention can be used for media with alarge cross section. In particular, there are no limitations withrespect to the lateral dimensions, as is, for example, the case withcylindrical mirrors. Spherical mirrors are less expensive to manufactureand easier to obtain and, therefore, the manufacturing costs of theinventive resonator are reduced. Cylindrical mirrors are optimized withrespect to a rectangular medium cross section. The inventive resonatorcan be used with any medium cross sections. In particular, adaptation toany medium cross sections can be carried out by using a scraper.

Owing to the asymmetrical outcoupling, a more compact outcouplingsurface (with smaller divergence) is made available, which provides fora far field with a reduced structure. In particular, an asymmetricalconfocal unstable negative-branch resonator is made available inaccordance with the invention.

A concave mirror surface of the back mirror advantageously faces theoptically active medium. Furthermore, a concave mirror surface of theoutcoupling mirror faces the optically active medium. A confocalresonator is thereby provided. This resonator is, in turn, unstable inall directions. Such a resonator is less sensitive to adjustment andalso less sensitive to deviations in mirror radii and tiltings than, forexample, a resonator with cylindrical mirrors.

The focal points of the back mirror and the outcoupling mirroradvantageously lie between these two mirrors. Provision is made for thecenter point of a sphere (an imaginary sphere) for the back mirror tolie on the optical axis. The focal point of the back mirroradvantageously lies on the optical axis between the back mirror and theoutcoupling mirror. It is also advantageous for the center point of asphere (an imaginary sphere) for the outcoupling mirror to lie on theoptical axis. For the above-mentioned same reason, it is advantageousfor the focal point of the outcoupling mirror to lie on the optical axisbetween the back mirror and the outcoupling mirror. It is particularlyadvantageous for the focal points of outcoupling mirror and back mirrorto coincide. A negative-branch resonator is then provided. An optimizedguidance of the radiation in the resonator is thereby achieved, and, inparticular, radiation can be coupled out parallel to the optical axis inan outcoupling region.

Optimum outcoupling is achieved when the common focal point lies closerto the outcoupling mirror than to the back mirror. Radiation whichoriginates from a certain region of the outcoupling mirror and isreflected at the back mirror can thereby be “fanned out”, i.e., thereturn reflection region at the back mirror is larger than the startingregion at the outcoupling mirror. In turn, radiation can thereby becoupled out in a simple way. The outcoupling can take place via acompact area (with “maximum” connectivity) and, in turn, a morehomogenous distribution of intensity in the far field is thereby madepossible.

It is advantageous for the radius of the back mirror to differ from theradius of the outcoupling mirror. In turn, optimized outcoupling isthereby obtained. It is particularly advantageous for the radius of theback mirror to be larger than the radius of the outcoupling mirror. Theoptimum ratio of the radii depends, inter alia, on the optically activemedium. In an exemplary embodiment, a ratio of the radii of between 1.1and 1.2 was chosen. However, this ratio can also be smaller or larger.

To obtain optimized laser activity, the lateral dimensions of the backmirror are advantageously larger than the corresponding lateraldimensions of the optically active medium and/or of a receptacle for theoptically active medium. The lateral dimensions are the dimensions inthe directions transverse to the optical axis. It is then ensured thatradiation within the resonator will pass through the optical medium.With gas as optically active medium, the receptacle in which the gas isaccommodated or through which the gas flows will define the lateraldimensions of the back mirror.

For the same reason, it is advantageous for the lateral dimensions ofthe outcoupling mirror to be larger outside of an outcoupling regionthan the corresponding lateral dimensions of the optically active mediumand/or of the receptacle for the optically active medium. Radiation iscoupled out via the outcoupling region, so there must be no returnreflection here.

It is advantageous for a outcoupling region to be of asymmetricalconfiguration in relation to the optical axis. A compact surface isthereby made available for the outcoupling region, whereby anadvantageous mode distribution with a time-independent mode and a farfield is obtained, which in comparison with known unstable resonatorswith small outcoupling has a less highly structured distribution ofintensity.

The asymmetrical outcoupling can be carried out by the outcouplingmirror being correspondingly shaped and/or the beam path within theresonator being correspondingly influenced. An effective mirror regionof the outcoupling mirror is asymmetrical in relation to the opticalaxis, and this effective mirror region is produced by shaping the mirroror by influencing the beam path.

In particular, an outcoupling region is associated with the outcouplingmirror. Radiation is coupled out of the resonator via this outcouplingregion. The outcoupling region is advantageously spaced from the opticalaxis. An optimized outcoupling is thereby achieved, and the inventiveresonator is then also usable together with optically active media withlarge diameter and/or small gain. Provision is made for the outcouplingregion to be designed in accordance with the power to be coupled out. Inparticular, the geometrical shape of the outcoupling region and/or thearrangement of the outcoupling region are such that the desired power iscoupled out. Provision may be made for the outcoupling mirror itself tobe designed such that an asymmetrical outcoupling takes place. Forexample, in a spherical mirror a half-ring region is cut out, and thiscut-out half-ring region then defines the outcoupling region.

Alternatively or additionally, provision may also be made for a scraperto be arranged between back mirror and outcoupling mirror. The scraperdefines in its arrangement and design the radiation component whichreaches the outcoupling mirror and which is reflected outwards at thescraper, i.e., is coupled out. A scraper is a mirror which has anaperture through which radiation can pass, and a mirror surface which,in particular, is arranged at or around the aperture and via which theradiation can be coupled out. The combination of outcoupling mirror andscraper then defines the outcoupling region. In particular, the scraperis arranged and designed such that an asymmetrical outcoupling will takeplace. For example, the scraper is only arranged on one side withrespect to a plane containing the optical axis. In particular, a mirrorsurface of the scraper defines an outcoupling region.

A mirror surface of the scraper is advantageously adapted to the opticalmedium and/or to a receptacle for the optical medium. Radiation canthereby be coupled out in a defined manner and, in particular, with adefined power. Provision may be made for the scraper to be arrangedsubstantially at a 45° angle to the optical axis. Radiation can therebybe coupled out of the resonator at a right angle to the optical axis.

For the asymmetrical outcoupling, there is advantageously associatedwith the outcoupling mirror a first mirror region which lies on the oneside of a plane containing the optical axis, and there is associatedwith the outcoupling mirror a second mirror region which lies on theother side of this plane, with the two mirror regions being of differentdesign. An asymmetry is thus made available, which provides for theasymmetrical outcoupling. The two mirror regions can be directly formedon the mirror or these can be effective mirror regions which areadjusted by influencing the beam, for example, by means of a scraper.

In particular, the first mirror region has a larger mirror surface thanthe second mirror region so as to obtain an asymmetrical outcoupling.Provision may be made for an outcoupling region to surround the secondmirror region. Furthermore, provision may be made for the first mirrorregion and the second mirror region to be joined in the plane containingthe optical axis. In particular, the second mirror region has smallerlateral dimensions than the first mirror region. An asymmetry is thusmade available, which, in turn, allows an asymmetrical outcoupling withthe corresponding advantages.

In order to obtain an asymmetrical outcoupling in relation to an axistransverse and, in particular, perpendicular to the optical axis, thesecond mirror region is advantageously of axially symmetrical design inrelation to such an axis. The first mirror region is also advantageouslyof axially symmetrical design in relation to such an axis. In relationto an axis lying transversely to this axis and transversely to theoptical axis, the second mirror region is of asymmetrical design.

In one embodiment the first mirror region is semicircular with respectto its outer circumference. Such a mirror region can be produced in asimple way. Provision may also be made for the second mirror region tobe semicircular with respect to its outer circumference. However, othergeometrical shapes are also possible. The shaping of the second mirrorregion does not necessarily have to be effected mechanically. If thesecond mirror region is an effective mirror region which is defined byirradiation, then an aperture in a scraper defines the shaping of thesecond mirror region. Adaptation to the cross section of the opticallyactive medium is possible via a scraper.

Provision is made for a projection of the optically active mediumparallel to the optical axis onto the back mirror to lie within anilluminated region of the back mirror. The projection of the opticallyactive medium can be a projection of the medium itself, the projectionof a receptacle for the optically active medium or the projection of aflow space for the optically active medium onto the back mirror. It isthen ensured that radiation from the back mirror (running parallel tothe optical axis) goes through the optically active medium.

The resonator according to the invention can be used with advantage in alaser. It is advantageous for a laser device and, in particular, a gaslaser device to comprise a resonator according to the invention.Provision may then be made for a flow space to be arranged between theback mirror and the outcoupling mirror for gas to flow therethrough. Ahigh laser power is thereby achievable.

The following description of preferred embodiments serves in conjunctionwith the drawings to explain the invention in greater detail.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention are explained in moredetail hereinbelow with reference to the drawings, which are notnecessarily drawn to scale, and wherein:

FIG. 1 shows a schematic perspective representation of a firstembodiment of a resonator in accordance with the invention;

FIG. 2 shows a sectional representation of the resonator according toFIG. 1 in a plane containing the optical axis and the axes y₁, y₂;

FIG. 3 shows a second embodiment of a resonator in accordance with theinvention;

FIG. 4 shows a sectional representation of the resonator according toFIG. 3 in a plane containing the optical axis and the axes y₁, y₂;

FIG. 5 shows a calculated distribution of intensity in the far field fora laser device with the resonator according to FIG. 1 or FIG. 3;

FIG. 6 shows a calculated distribution of intensity in the near fieldfor a laser device with a resonator according to FIG. 1 or FIG. 3;

FIG. 7 shows a schematic representation of a first embodiment of a gaslaser device; and

FIG. 8 shows a schematic representation of a second embodiment of a gaslaser device.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter.However, this invention may be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein;rather, these embodiments are provided so that this disclosure willsatisfy applicable legal requirements. Like numbers refer to likeelements throughout.

A first embodiment of an unstable optical resonator according to theinvention, which is shown in FIGS. 1 and 2 and is generally designatedtherein by reference numeral 10, comprises a back mirror 12 and anoutcoupling mirror 14. The two mirrors 12 and 14 are spaced from eachother, and the optically active medium 16 is arranged or guided betweenthese mirrors. In particular, in the case of a gas laser, a receptacle,which accommodates the optically active medium, or a flow space, throughwhich the optically active medium flows, is arranged between the mirrors12, 14. Such a receptacle or container is shown in FIG. 7 and designatedtherein by the reference numeral 70. A flow space 84 is shown in FIG. 8.

The back mirror 12 is of spherical design with a concave surface 18,which faces the optically active medium 16. This concave surface 18 hasa certain radius R₂. The concave surface 18 lies on the surface of animaginary sphere, and the radius is measured with respect to the centerpoint of the sphere. The imaginary center point of the sphere lies on anoptical axis 20.

The outcoupling mirror 14 is likewise of spherical design with aspherical, concave surface 22, which faces the optically active medium16. The two concave surfaces 18 and 22 thus face each other. The concavesurface 22 lies on the surface of an imaginary sphere, and the centerpoint of this imaginary sphere lies on the optical axis 20. Thisimaginary sphere has a radius R₁, which corresponds to the radius of theconcave surface 22.

The radius R₂ of the back mirror 12 for its concave surface 18 is largerthan the radius R₁ of the outcoupling mirror 14 for its concave surface22. In an exemplary embodiment, the ratio of the radius of the backmirror 12 to the radius of the outcoupling mirror 14 (R₂/R₁) is between1.1 and 1.2. However, smaller ratios (greater than 1) or larger ratiosare also possible.

The back mirror 12 has a focal point 24 which lies on the optical axis20. The outcoupling mirror 14 likewise has a focal point which lies onthe optical axis. The two mirrors 12 and 14 are arranged and designedsuch that their focal points substantially coincide. The focal pointdesignated by reference numeral 24 in FIG. 2 is then the focal point ofboth back mirror 12 and outcoupling mirror 14. The coinciding focalpoint 24 of the two mirrors 12, 14 preferably lies closer to theoutcoupling mirror 14 than to the back mirror 12.

The back mirror 12 has larger lateral dimensions (transversely to theoptical axis 20), so that it covers the optically active medium 16 orthe receptacle for the optically active medium. Provision may be madefor the lateral dimensions of the back mirror 12 to be larger in orderto, for example, take diffraction losses into consideration.

A projection of the optically active medium in a direction parallel tothe optical axis 20 onto the back mirror 12 lies within an illuminatedregion of the back mirror 12. The illuminated region is defined by thespacing between the back mirror 12 and the outcoupling mirror 14 and bythe radii of these two mirrors 12, 14 and the design of the outcouplingmirror 14 (or a scraper, as will be described in greater detailhereinbelow). With a given optical medium or receptacle for the opticalmedium or flow space for the optical medium, the mirrors 12, 14 are thento be designed in such a way that the optical medium does not projectbeyond the illuminated region of the back mirror 12 and is therefore notilluminated in a projecting region.

Since the radiation emanating from the back mirror 12 is orientedparallel to the optical axis 20, the projection of the optically activemedium parallel to the optical axis 20 must lie within the illuminatedregion. With the exception of an outcoupling region, which will beexplained in greater detail hereinbelow, the outcoupling mirror 14likewise has such lateral dimensions that the optically active medium 16is covered transversely to the optical axis 20.

The resonator 10 is a confocal optical resonator with negative branch,i.e., the focal point 24 lies between the mirrors 12 and 14.Furthermore, the resonator 10 is asymmetrical in relation to the opticalaxis 20, i.e., an asymmetrical optical outcoupling takes place. Such anasymmetrical outcoupling is caused by an asymmetry in the beam pathbetween the two mirrors 12 and 14.

In the embodiment according to FIGS. 1 and 2, there is associated withthe outcoupling mirror 14 an outcoupling region 26, which is broughtabout by corresponding design of the outcoupling mirror 14. To this end,the outcoupling mirror 14 has a first mirror region 28 and a secondmirror region 30. The two mirror regions 28, are integrally joined andcomprise the concave surface 22. The first mirror region 28 lies on oneside 32 of a plane 34 which contains the optical axis 20 and the axesx₁, x₂. The second mirror region 30 lies on the other side 36 of thisplane 34. The two mirror regions 28, 30 are joined together in the plane34.

The second mirror region 30 has a smaller surface than the first mirrorregion 28. The second mirror region 30 is therefore different in designto the first mirror region 28. Relative to the x₁ axis, it is set backfrom the second mirror region 30 and is likewise set back relative tothe y₁ axis. The height of the second mirror region 30 in direction y₁is therefore smaller than the height of the first mirror region 28 inthe same direction.

In the illustrated exemplary embodiment, the first mirror region 28 isof semicircular design with respect to its outer contours. The secondmirror region 30 is likewise of semicircular design, but has a smallerradius than the first mirror region 28. Here, radius means radius of acircle and not radius R₁ of the imaginary sphere for the concave surface18. The outer dimensions of the first mirror region 28 correspond to theouter dimensions of an opposite region of the back mirror 12. The regionof the back mirror 12 located opposite the second mirror region 30 is,however, larger than the second mirror region 30. Other designs are alsopossible for the mirror region. For example, the second mirror regioncan extend into the half-space 32. In the illustrated embodiment theoutcoupling region 26 surrounds the second mirror region 30 in the shapeof a half ring.

In the unstable confocal negative-branch resonator 10, radiationemanating from a region comprising the second mirror region 30 and itscircular continuation into the first mirror region 28 impinges on theback mirror 12 after passing through the focal point 24. The radiationreflected in a corresponding region at the back mirror 12 travelsparallel to the optical axis 20 to the outcoupling mirror 14. Thereflection region at the back mirror 12 is larger than the “startingregion” of the outcoupling mirror 14, as the back mirror 12 has a largerradius R₂ and the focal point 24 lies closer to the outcoupling mirror14. The radiation reflected by the back mirror 12 then impinges again onthe outcoupling mirror 14 in a region which is larger than the secondmirror region 30. In particular, this region comprises a half-ringsurface at the first mirror region 28 on the side 32 of the plane 34.The diameter of this ring region corresponds to the diameter of thereturn reflection region in the back mirror 12. The radiation coming inon the side 36, which does not impinge on the second mirror region 30,i.e., lies in the outcoupling region 26, leaves the resonator 10.

The radiation impinging on the outcoupling mirror 14 is reflected and,after passing through the focal point 24, reaches the back mirror 12again. This passes into the above-mentioned return reflection region ifthe radiation originates from the second mirror region 30. The radiationwhich originates from the above-mentioned half-ring region impinges on aregion of the back mirror 12 adjacent to this return reflection region,which lies on the side 36 of the optical axis. The region lying belowthe optical axis 20 (i.e., the region lying on the side 32), which liesoutside the return reflection region, is not illuminated. The radiationthen reflected again from the back mirror 12 impinges on the outcouplingmirror 14, and part of the radiation reflected at the back mirror 12reaches the outcoupling region 26 and leaves the resonator 10.

With the design of the outcoupling mirror 14 shown, radiation leaves theresonator 10 in a semicircular region (disregarding diffraction). Thecoupled out power can be adjusted by corresponding adjustment of theoutcoupling region, i.e., by corresponding adjustment of the mirrorregion 30.

In a second embodiment of an unstable confocal negative-branch resonatoraccording to the invention, which is shown schematically in FIGS. 3 and4 and generally designated therein by reference numeral 38, the backmirror is basically of the same design as described hereinabove. Thesame reference numerals are therefore used therefor. The back mirror 12is again arranged symmetrically in relation to the optical axis 20. Anoutcoupling mirror 40 is provided. This again has a spherically concavesurface 42 facing an optical medium 44 or a receptacle or flow space foran optical medium. The outcoupling mirror 40 is of symmetrical designwith a radius (based on an imaginary sphere, the surface 42 of whichrepresents a surface region) which is smaller than the radius of acorresponding imaginary sphere for the back mirror 12. The focal pointof the two confocal mirrors 12 and 40 coincides (negative branch), andthis focal point preferably lies closer to the outcoupling mirror 40.

For the asymmetrical outcoupling of radiation, a scraper 46 is arrangedbetween the two mirrors 12 and 40. This scraper is an element with amirror surface 48 at which radiation is reflected so as to be coupledout of the resonator 38. Furthermore, the scraper 46 has an aperture 50permeable to radiation, through which radiation reflected from the backmirror 12 can pass to the outcoupling mirror 40. The aperture 50 issmaller than the mirror surface of the outcoupling mirror 40.

The optical axis 20 defines a plane 52 with a side 54 and an oppositeside 56. The plane 52 separates the two sides 54, 56. The sides 54 and56 are therefore half-spaces. The scraper 46 is seated, for example, onone side, for example, on side 56. It does not influence the beam pathon the side 54. It is also possible for the scraper to extend beyond theplane containing the optical axis 20. The aperture of the scraper isthen of corresponding design.

The mirror surface 48 of the scraper 46 is also arranged and designedsuch that in its transverse dimensions (in the x₁-y₁ plane) it is atleast as large as the optically active medium 44 or its receptacle inthe region 58 of the optically active medium 44 or its receptacle thatfaces the scraper 46, with the exception of the aperture 50. Forexample, the scraper 46 is arranged at an angle of 45° to the opticalaxis 20. Thus, radiation can be coupled out of the resonator 38 parallelto the optical axis 20 at right angles to the optical axis 20.

Owing to the scraper 46 with its aperture 50 there is associated withthe outcoupling mirror 40 a first mirror region 60 and a second mirrorregion 62, with the aperture 50 defining the second mirror region 62.The first mirror region 60 corresponds to the first mirror region 28 inresonator 10. The second mirror region 62 corresponds to the secondmirror region 30 in resonator 10. The second mirror region 62 in theresonator 38 is not obtained by the design and, in particular, shapingof the outcoupling mirror, but by the arrangement and shaping of thescraper 46. An outcoupling region 64 for radiation from the resonator 38is defined by the design and arrangement of the mirror surface 48 of thescraper 46. The resonator 38 is an asymmetrical, confocal, unstableoptical negative-branch resonator.

The aperture 50 can be adapted to the optically active medium 44 or areceptacle. For example, with a round laser rod a round aperture will beselected, whereas with a rectangular receptacle or rectangular laser roda rectangular aperture 50 will be selected. With fixed mirrors 12, 40, adesired outcoupling, in particular, with respect to the desiredoutcoupling region can then be adjusted by corresponding choice of ascraper 46 with aperture 50 and mirror surface 48. Furthermore,adaptation to the optically active medium 44 is possible in a simpleway.

In the preferred embodiments shown, the mirrors 12, 14 and 40 are ofcircular or semicircular shape with respect to their outer dimensions.Other shapes are also possible.

In accordance with the invention, an unstable optical resonator withspherical mirrors 12, 14 and 12, 40, respectively, is provided. It is apurely unstable resonator without stable direction. The problems thatoccur in conjunction with stable resonators are thus avoided. Inparticular, the formation of a high multimode with an intensitydistribution varying with respect to time is avoided. High resonatorlosses occur with cylindrical mirrors when the mirror is narrow in theplane (stable) direction. Furthermore, cylindrical mirrors are verysensitive to adjustment in stable direction (as planar mirror surfacesare located opposite). With the solution according to the invention withspherical mirrors 12, 14 and 12, 40, respectively, the tolerancesrequired for resonator operation are easier to achieve. The resonator istherefore easier to construct. Moreover, spherical mirrors are morefavorably priced and easier to obtain than cylindrical mirrors. Inparticular, adjustment of the mirrors 12, 14 and 12, 40, respectively,is easier than, for example, with cylindrical mirrors, and the resonatorin accordance with the invention is less sensitive to deviations inmirror radii and tiltings in comparison with cylindrical mirrors.

By using a scraper, it is also possible to carry out an adaptation of,for example, round or rectangular cross sections for the opticallyactive medium. In contrast, cylindrical mirrors are only optimal forrectangular medium cross section. With the solution according to theinvention with spherical mirrors 12, 14 and 12, 40, respectively, thereare no limitations with respect to the width as with cylindricalmirrors. A width which is too small in the stable direction causes ahigh resonator loss in the case of cylindrical mirrors, whereas a widthwhich is too large may result in a mode which is variable with respectto time.

FIGS. 5 and 6 show field diagrams for the resonators 10 and 38,respectively. FIG. 5 shows the calculated distribution of intensity inthe far field with respect to angles relative to the x₁ axis and y₁axis. When plotted over these divergence angles, an independence withrespect to the spacing from the outcoupling mirror is achieved. Compactdistribution of the intensity in the far field is evident.

FIG. 6 shows the calculated distribution of intensity in the near fieldin the x₁-y₁ plane. The optical axis passes through the point 0-0. Theinfluence on the near field by a corresponding shaping of theoutcoupling regions 26 and 64, respectively, is evident.

The inventive resonators can be used with advantage together withoptically active media which have a small gain and require a large crosssection. A time-independent mode and in the far field an intensitydistribution which is not too highly structured are thereby achievable.In particular, the solution according to the invention may be usedtogether with gas laser devices.

FIG. 7 shows schematically a first embodiment of a gas laser device 66.This comprises an inventive resonator 68, which is basically designed asdescribed hereinabove with reference to the first embodiment 10 or thesecond embodiment 38.

A receptacle 70 for the optically active gas medium is arranged in theresonator 68. A gas is conducted in a flow guide 72 through thisreceptacle 70. A pump 74 is provided for this purpose. A gas reservoir75 is arranged in the flow guide 72. The direction of flow is indicatedby the arrow designated by reference numeral 76. The output power of thelaser device 66 can be increased by a longitudinal gas flow in thereceptacle 70. The gas is excited by an applied high voltage (indicatedby reference numeral 78 in FIG. 7).

A resonator 82 according to the invention is again provided in a secondembodiment of a gas laser device shown schematically in FIG. 8 andgenerally designated therein by reference numeral 80. A flow space 84for gas flowing therethrough is arranged in the resonator 82. A highvoltage is applied (electrodes are indicated by reference numeral 86 inFIG. 8) to excite the gas. The gas flows through the flow space 84transversely to the direction of the radiation (i.e., transversely tothe optical axis 20). In the example shown, the discharge takes placetransversely to the laser beam and the gas likewise flows transverselyto the laser beam. Very high output powers are thereby achievable.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing description. Therefore, it is to be understood that theinventions are not to be limited to the specific embodiments disclosedand that modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

1. Unstable optical resonator for an optically active medium,comprising: a spherical back mirror; a spherical outcoupling mirror; anda scraper being arranged between the back mirror and the outcouplingmirror; wherein the scraper is arranged and designed such that anasymmetrical outcoupling will take place in relation to the optical axiswith an outcoupling region being of asymmetrical design in relation tothe optical axis; wherein the optical resonator is purely unstable; andwherein the scraper has a mirror surface which, in a plane perpendicularto the optical axis, defines outer dimensions at least as large as thecorresponding outer dimensions of the optically active medium or areceptacle for the optically active medium in this plane.
 2. Unstableoptical resonator in accordance with claim 1, wherein a concave mirrorsurface of the back mirror faces the optically active medium. 3.Unstable optical resonator in accordance with claim 1, wherein a concavemirror surface of the outcoupling mirror faces the optically activemedium.
 4. Unstable optical resonator in accordance with claim 1,wherein the focal points of the back mirror and the outcoupling mirrorlie between these two mirrors.
 5. Unstable optical resonator inaccordance with claim 1, wherein a center point of a sphere for the backmirror lies on the optical axis.
 6. Unstable optical resonator inaccordance with claim 5, wherein the focal point of the back mirror lieson the optical axis between the back mirror and the outcoupling mirror.7. Unstable optical resonator in accordance with claim 1, wherein acenter point of a sphere for the outcoupling mirror lies on the opticalaxis.
 8. Unstable optical resonator in accordance with claim 7, whereinthe focal point of the outcoupling mirror lies on the optical axisbetween the back mirror and the outcoupling mirror.
 9. Unstable opticalresonator in accordance with claim 1, wherein the focal points ofoutcoupling mirror and back mirror coincide.
 10. Unstable opticalresonator in accordance with claim 9, wherein the common focal pointlies closer to the outcoupling mirror than to the back mirror. 11.Unstable optical resonator in accordance with claim 1, wherein theradius of the back mirror differs from the radius of the outcouplingmirror.
 12. Unstable optical resonator in accordance with claim 11,wherein the radius of the back mirror is larger than the radius of theoutcoupling mirror.
 13. Unstable optical resonator in accordance withclaim 1, wherein the lateral dimensions of the back mirror are largerthan the corresponding lateral dimensions of at least one of theoptically active medium and a receptacle for the optically activemedium.
 14. Unstable optical resonator in accordance with claim 1,wherein the lateral dimensions of the outcoupling mirror outside of anoutcoupling region are larger than the corresponding lateral dimensionsof at least one of the optically active medium and a receptacle for theoptically active medium.
 15. Unstable optical resonator in accordancewith claim 1, wherein an outcoupling region is of asymmetricalconfiguration in relation to the optical axis.
 16. Unstable opticalresonator in accordance with claim 15, wherein an effective mirrorregion of the outcoupling mirror is asymmetrical in relation to theoptical axis.
 17. Unstable optical resonator in accordance with claim15, wherein an outcoupling region is associated with the outcouplingmirror.
 18. Unstable optical resonator in accordance with claim 17,wherein the outcoupling region is spaced from the optical axis. 19.Unstable optical resonator in accordance with claim 17, wherein theoutcoupling region is configured in accordance with the power to becoupled out.
 20. Unstable optical resonator in accordance with claim 1,wherein a mirror surface of the scraper defines an outcoupling region.21. Unstable optical resonator in accordance with claim 1, wherein amirror surface of the scraper is adapted in design to at least one ofthe optically active medium and a receptacle for the optically activemedium.
 22. Unstable optical resonator in accordance with claim 1,wherein the scraper is arranged substantially at a 45° angle to theoptical axis.
 23. Unstable optical resonator in accordance with claim 1,wherein a projection of the optically active medium parallel to theoptical axis onto the back mirror lies within an illuminated region ofthe back mirror.
 24. Gas laser device, comprising: an unstable opticalresonator for an optically active medium, said resonator having aspherical back mirror; a spherical outcoupling mirror; and a scraperbeing arranged between the back mirror and the outcoupling mirror;wherein the scraper is arranged and designed such that an asymmetricaloutcoupling will take place in relation to the optical axis with anoutcoupling region being of asymmetrical design in relation to theoptical axis; wherein the optical resonator is purely unstable; andwherein the scraper has a mirror surface which, in a plane perpendicularto the optical axis, defines outer dimensions at least as large as thecorresponding outer dimensions of the optically active medium or areceptacle for the optically active medium in this plane.
 25. Gas laserdevice in accordance with claim 24, wherein a flow space for gas to flowtherethrough is arranged between the back mirror and the outcouplingmirror.